patch to C4::Members::ModMember to prevent it from deleting the dateofbirth field when none is supplied.
I also added a KohaTest::random_date method to help generate randomish dates for the test suite.
Added some tests for Member::ModMember. This is an easy method to test, and this bug shows that it
could use some closer examiniation.
Signed-off-by: Joshua Ferraro <jmf@liblime.com>